This invention relates generally to microstructures and manufacturing methods and more particularly to microbolometers and methods of manufacturing such devices.
As is known in the art, semiconductor manufacturing techniques are being applied to form various microstructures. One such microstructure is a microbolometer used to detect infrared energy. More particularly, an array of such devices is formed integrally with an integrated circuit. The integrated circuit is used to develop electrical signals produced by the microbolometers in .response to the infrared energy impinging on the array thereof. In one such array, each one of the microbolometers includes an infrared energy receiving surface. The surface is made of a material which has its resistivity change as the temperature of the material increases in response to the infrared energy impinging on, and absorbed by, the material. One such material is vanadium oxide. Thus, by placing the material in series with a voltage supply, the current through the material will vary in accordance with the infrared energy sensed by the material. Electronic read circuitry connected to the voltage supply and serially connected temperature sensitive resistive material is used to produce an output signal representative of the infrared energy impinging on the material. The array of bolometers, together with its output electrical signals, and a processor fed by the output electrical signals can thus be used to provide an electronic image of the source of the infrared energy.
As noted above, in some applications, integrated circuit fabrication techniques have been used to form the array of bolometers integrally with the associated electronic read out circuitry. In such application, a substrate, or semiconductor layer, has formed, as integrated circuit, the read out electronics. More particularly, the read out electronics includes an array of read out cells, each one thereof corresponding to one of the bolometers in the array. Each read out cell includes a pair of electrical contacts formed on the upper surface of the semiconductor layer for electrical connection to a corresponding one of the bolometers in the array thereof. Each one of the bolometers includes a thermal insulating, dielectric platform integrally formed over the semiconductor layer. The platform, typically silicon nitride, is formed subsequent to the formation of the array of read out cell. The platform is formed, using conventional integrated circuit photolithography, i.e., sequential photolithographic masking and patterning of layers of material by chemical etching and/or liftoff techniques. Using such techniques, the platform is formed with a pair of legs with proximal ends disposed on the surface at positions adjacent the electrical contacts. The legs are formed so that the distal ends of the legs are elevated over the surface of the substrate. Integrally formed with the legs is a planar square, or rectangular shaped surface member. Opposing corners of the surface member terminate into the distal end of the pair of legs. Thus, the surface member is suspended by the legs, as an air bridging surface, over the surface of the substrate. The temperature sensitive resistive material is formed over the surface member. Electrical conductors are formed over the legs, proximal ends of the conductors being in contact with the electrical contacts and the distal ends being electrically connected to the portions of the temperature sensitive resistive material adjacent the distal ends of the legs. Such arrangement suspends the temperature sensitive resistive material on the elevated, air-bridging surface member from the substrate to thereby increase its thermal isolation from the substrate. The increased thermal isolation thereby increases the sensitivity of the bolometer to the impinging infrared energy. While the suspended, air-bridging, surface member increases the thermal isolation (compared with an arrangement where the temperature sensitive resistive material is in physical contact with the substrate) the temperature sensitive resistive material is not totally isolated because of heat transfer through the legs.